Roots and endings
Three conditions hold up everything and are held up by nothing. Three others hold up nothing at all. Between those two lists sits everything else.
Every dependency graph has two interesting edges. There are the nodes with nothing behind them, which is where explanations have to stop. And there are the nodes with nothing in front of them, where consequences stop travelling. On this map there are three of each, which is a small enough number to look at directly.
Why the roots matter more than they look
A root condition has no upstream, so when it is false there is no further diagnosis to do. That sounds convenient and is not, because it also means nothing on the map warned you. The three roots here are the machine, the room and the decision to hold an address as characters rather than as a click. All three are things you brought with you before the first page loaded.
Count the arrows out of a root and you get its blast radius. One of them is directly upstream of four other conditions and indirectly upstream of most of the map. Another carries a single arrow. They are both roots, and they are not remotely equally expensive.
Why the endings matter too
A terminal condition is one nothing else waits on. It is tempting to read that as unimportant and it is not what it means. It means containment. When a terminal condition is false you have a real and often immediate problem, and it stays where it is instead of propagating. Given a fixed amount of attention, that is a genuinely useful thing to know about a failure.
The three endings on this map are a browser kept separate from everything else, a name that fits the door a parcel arrives at, and being able to come back tomorrow. The last one is worth a second look, because it is terminal on the map and yet failing it sends you back to stage C to rebuild the whole chain. Terminal means nothing depends on it here. It does not mean nothing follows from it in life.
Read the two lists below together. The first is where failures begin. The second is where they stop. Everything else on the map is somewhere in between, passing consequences along.
The root conditions
These depend on nothing else on the map. Everything else eventually rests on them, so if one of them is false the failure travels a long way.
The conditions that end a chain
Nothing on this map waits on these. A failure here is expensive in itself, but it does not knock anything else over.